The present invention generally relates to a device for producing a gaseous measuring sample from a sample liquid in a sample vessel for transfer into the flame or measuring cuvette of an atomic absorption spectrometer, and, in particular, relates to such a device including means for equalizing the differential pressure across the reagent dosing conduit to interrupt the adding of reagent.
The production of volatile hydrides of a sought element, e.g. arsenic or selenium, in a sample vessel by adding a strong reducing agent, such as a reagent, thereto is known. The volatile hydrides so produced are then transferred to a heated measuring cuvette of an atomic absorption spectrometer via an inert gas flow and are therein thermally decomposed. The sought element thus appears in its atomic state in the measuring cuvette, and its atomic absorption may be measured. It is also possible to release a volatile substance from its compounds by the reagent, which volatile substance is carried away by the inert gas flow such that the atomic absorption of this volatile substance may be determined in the measuring cuvette.
In order to perform this analysis an inert gas flow is initially passed through the sample vessel and the measuring cuvette via an inert gas feed conduit. These components are flushed by the inert gas, and the air is displaced from the system. Subsequently, a reagent, e.g. NaBH.sub.4, is added to the sample liquid, whereby a volatile measuring sample e.g. AsH.sub.3, is produced. This volatile measuring sample is transferred to the heated measuring cuvette by the inert gas flow and is thermally decomposed therein such that an "atomic cloud" of the sought element is formed.
One such known reagent adding device (German Auslegeschrift No. 27.18.381 corresponding to Huber, U.S. Pat. No. 4,208,372) contains a gaseously sealed reagent reservoir. A connecting conduit leads from an inert gas feed conduit to the reagent reservoir and permits a build up of pressure therein. A reagent dosing conduit leads from the bottom of the reagent reservoir to the sample vessel. If the stop valve in the inert gas feed conduit is closed, reagent liquid is displaced from the reagent reservoir by the inert gas pressure and is pressed into the sample vessel through the reagent dosing conduit. If the stop valve is opened, pressure equalization between sample vessel and reagent reservoir is effected such that the delivery of reagent is interrupted.
The German Auslegeschrift No. 27.35.281 (corresponding to Huber, U.S. Pat. No. 4,208,372) shows one arrangement for ensuring a good mixing of reagent and sample liquid by leading the reagent into the lower part of the sample vessel. Of course, measures are taken to prevent the reagent liquid from continuously flowing or to prevent the sample from flowing back into the reagent reservoir.
From the German Offenlegungsschrift No. 27.29.744 (corresponding to Huber, U.S. Pat. No. 4,230,665) a device is known which permits automatic analysis of a series of sample liquids one-by-one in producing a gaseous sample from each of these sample liquids and measuring it by measuring the atomic absorption.
The individual samples are introduced sequentially into a reaction vessel by means of a sampler. A reagent is added each time by means of a reagent supply device. The resulting gaseous measuring sample is transferred to a heatable measuring cuvette through a carrier gas discharge conduit. The bottom of the reaction vessel includes a valved drain through which the contents of the reaction vessel is discharged into a waste vessel upon completion of the measurement. A program control is arranged to control the reagent supply device, the sampler and the drain valve in accordance with a preset program which program includes a measuring cycle for each sample.
From the German Auslegeschrift No. 28.51.058 (corresponding to Huber et al, U.S. Pat. No. 4,273,742) a modification of this device is known which ensures that the program proceeds to the next measuring cycle only if the reaction vessel or sample vessel is completely empty. To this end, the drain valve is a relief valve which opens in the discharge direction but is biased in the closing direction. A controlled stop valve is located in the carrier gas discharge conduit to the measuring cuvette. A pressure sensor is connected to the carrier gas supply conduit and responds to a predetermined pressure threshold being exceeded. The program control device is arranged to close the stop valve in the carrier gas discharge conduit after the measurement has been made, such that the reaction, or sample vessel, is gaseously sealed. The program control device is arranged to proceed to the next measuring cycle, if the pressure sensor is in its unactuated state after a predetermined time interval. The unactuated state corresponds to a pressure below a preselected threshold pressure valve. After the stop valve in the carrier gas discharge conduit is closed a pressure builds up in the reaction or sample vessel, consequently the relief valve is urged open and the liquid is forced out of the reaction, or sample vessel, into waste vessel. Due to its viscosity, the liquid encounters a flow resistance during discharge which resistance is sufficiently strong that a further pressure increase occurs in the reaction or sample vessel during discharge of the liquid. Thereby, the pressure sensor responds. As soon as the reaction, or sample, vessel is completely emptied, only carrier gas flows through the drain. The pressure in the reaction, or sample, vessel breaks down due to the reduced flow resistance presented by the carrier gas to the drain. The pressure sensor, consequently, returns its unactuated state. The program proceeds to the next measuring cycle only, if this is the case after a preset time interval.